playButton.onclick = ()=> {
    console.log('timeline play()', tl);
    // play() only plays forward from current position. If timeline has finished, play() has nowhere to go.
    // if the timeline is no done then play() or else restart() (restart always restart from beginning).
    if (tl.progress() != 1) {
        tl.play();
    } else {
        tl.restart();
    }
};

restartButton.onclick = ()=> {
    tl.restart();
};

const restartAction = ()=> {
    toolTimeline.restart();
};